1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device having a level shifter for carrying out signal transmission between circuits operating at different power supply voltages.
2. Description of the Related Art
When an electrically rewritable non-volatile semiconductor memory device constructed by MOS transistors has a mixture of both circuits operating at a high voltage and at a low voltage, generally a gate oxide film of a MOS transistor for the high voltage circuit is formed thicker than that of a MOS transistor for the low voltage circuit to secure reliability. When a signal is transmitted from the high voltage circuit to the low voltage circuit, a circuit for shifting the level is required between the two circuits. One of the most widely used level shifter is shown in FIG. 4, in which an inverter circuit, which is composed of MOS transistors having a thick gate oxide film, is inserted between a high voltage circuit and a low voltage circuit, and is driven at a low power supply voltage (for example, refer to JP 10-242434 A (p.9, FIG. 3)).
Hereinafter, description will be made of a conventional semiconductor integrated circuit device having a level shifter with reference to FIG. 4.
A PMOS transistor 1 formed of a thick gate oxide film and an NMOS transistor 2 formed of a thick gate oxide film constitute an inverter 51 operating at a high voltage Vpp. A PMOS transistor 3 formed of a thin gate oxide film and an NMOS transistor 4 formed of a thin gate oxide film constitute an inverter 52 operating at a low voltage Vdd. An output signal from the inverter 51 enters to an input terminal of an inverter 53 composed of a PMOS transistor 5 formed of a thick gate oxide film and an NMOS transistor 6 formed of a thick gate oxide film. At this point, since a source electrode of the PMOS transistor 5 formed of a thick gate oxide film is connected to the low voltage Vdd, the output amplitude level ranges from Vdd to GND even when the amplitude level of the input ranging from Vpp to GND. The inverter 53 constituted by the PMOS transistor 5 formed of a thick gate oxide film and the NMOS transistor 6 formed of a thick gate oxide film works as a level shifter, and is, accordingly, able to-transmit an output signal from the high voltage circuit to the low voltage circuit.
When the number of level shifters mounted on a conventional semiconductor integrated circuit device is large, however, the area of the level shifters becomes large with respect to the area of the entire semiconductor integrated circuit device. Since size-reduction for the MOS transistor having a thick gate oxide film is more difficult than that for the MOS transistor having a thin gate oxide film, in the semiconductor integrated circuit device having a large number of level shifters it has been a problem that the area of its chip becomes extremely large, which leads to increased manufacturing cost.